The finite simple groups: an introduction

310 Pages·2009·3.746 MB·English

by  Wilson, Robert

251
Graduate Texts in Mathematics
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S.Axler
K.A.Ribet
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Robert A. Wilson
The Finite Simple Groups
ProfessorRobertA.Wilson
QueenMary,UniversityofLondon
SchoolofMathematicalSciences
MileEndRoad
LondonE14NS
UK
[email protected]
EditorialBoard
S.Axler K.A.Ribet
MathematicsDepartment MathematicsDepartment
SanFranciscoStateUniversity UniversityofCalifornia,Berkeley
SanFrancisco,CA94132 Berkeley,CA94720-3840
USA USA
[email protected] [email protected]
ISSN0072-5285
ISBN978-1-84800-987-5 e-ISBN978-1-84800-988-2
DOI10.1007/978-1-84800-988-2
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Preface
Thisbookisintendedasanintroductiontoallthefinitesimplegroups.During
themonumentalstruggletoclassifythefinitesimplegroups(andindeedsince),
a huge amount of information about these groups has been accumulated.
Conveyingthisinformationtothenextgenerationofstudentsandresearchers,
not to mention those who might wish to apply this knowledge, has become a
major challenge.
WiththepublicationofthetwovolumesbyAschbacherandSmith[12,13]
in 2004 we can reasonably regard the proof of the Classification Theorem for
Finite Simple Groups (usually abbreviated CFSG) as complete. Thus it is
timely to attempt an overview of all the (non-abelian) finite simple groups
in one volume. For expository purposes it is convenient to divide them into
four basic types, namely the alternating, classical, exceptional and sporadic
groups.
The study of alternating groups soon develops into the theory of permu-
tation groups, which is well served by the classic text of Wielandt [170] and
more modern treatments such as the comprehensive introduction by Dixon
and Mortimer [53] and more specialised texts such as that of Cameron [19].
The study of classical groups via vector spaces, matrices and forms encom-
passes such highlights as Dickson’s classic book [48] of 1901, Dieudonn´e’s [52]
of1955,andmoremoderntreatmentssuchasthoseofTaylor[162]andGrove
[72]. The complete collection of groups of Lie type (comprising the classical
and exceptional groups) is beautifully exposed in Carter’s book [21] using
the simple complex Lie algebras as a starting point. And sporadic attempts
have been made to bring the structure of the sporadic groups to a wider
audience—perhaps the most successful book-length introduction being that
of Griess [69]. But no attempt has been made before to bring within a sin-
gle cover an introductory overview of all the finite simple groups (unless one
counts the ‘Atlas of finite groups’ [28], which might reasonably be considered
to be an overview, but is certainly not introductory).
TheremitIhavegivenmyself,tocoverallofthefinitesimplegroups,gives
both advantages and disadvantages over books with more restricted subject
VI Preface
matter. On the one hand it allows me to point out connections, for example
betweenexceptionalbehaviourofgenericgroupsandtheexistenceofsporadic
groups.Ontheotherhanditpreventsmefromprovingeverythinginasmuch
detail as the reader may desire. Thus the reader who wishes to understand
everythinginthisbookwillhavetodoalotofhomeworkinfillingingaps,and
following up references to more complete treatments. Some of the exercises
are specifically designed to fill in gaps in the proofs, and to develop certain
topics beyond the scope of the text.
One unconventional feature of this book is that Lie algebras are scarcely
mentioned. The reasons for this are twofold. Firstly, it hardly seems possible
toimproveonCarter’sexposition[21](althoughthisbookisnowoutofprint
and secondhand copies change hands at astronomical prices). And secondly,
the alternative approach to the exceptional groups of Lie type via octonions
deserves to be better known: although real and complex octonions have been
extensively studied by physicists, their finite analogues have been sadly ne-
glected by mathematicians (with a few notable exceptions). Moreover, this
approach yields easier access to certainkey features, such as the orders of the
groups, and the generic covering groups.
On the other hand, not all of the exceptional groups of Lie type have had
effectiveconstructionsoutsideLietheory.InthecaseofthefamilyoflargeRee
groups I provide such a construction for the first time, and give an analogous
descriptionofthesmallReegroups.Theimportanceoftheoctonionsinthese
descriptions led me also to a new octonionic description of the Leech lattice
andConway’sgroup,andtoanambition,notyetrealised,toseetheoctonions
atthecentreoftheconstructionofalltheexceptionalgroupsofLietype,and
many of the sporadic groups, including of course the Monster.
Complete uniformity of treatment of all the finite simple groups is not
possible, but my ideal (not always achieved) has been to begin by describ-
ing the appropriate geometric/algebraic/combinatorial structure, in enough
detail to calculate the order of its automorphism group, and to prove sim-
plicity of a clearly defined subquotient of this group. Then the underlying
geometry/algebra/combinatoricsis further developed in order to describe the
subgroup structure in as much detail as space allows. Other salient features
of the groups are then described in no particular order.
This book may be read in sequence as a story of all the finite simple
groups, or it may be read piecemeal by a reader who wants an introduction
to a particular group or family of groups. The latter reader must however
be prepared to chase up references to earlier parts of the book if necessary,
and/or make use of the index. Chapters4 and 5 are largely (but not entirely)
independent of each other, but both rely heavily on Chapters 2 and 3. The
sections of Chapter 4 are arranged in what I believe to be the most appropri-
ateorderpedagogically,ratherthanlogicallyorhistorically,butcouldberead
in a different order. For example, one could begin with Section 4.3 on G (q)
2
and proceed via triality (Section 4.7) to F (q) (Section 4.8) and E (q) (Sec-
4 6
tion4.10), postponing the twistedgroupsuntil later.The orderingofsections
Preface VII
inChapter5istraditional,butamoreavant-gardeapproachmightbeginwith
J (Section 5.9.1), and follow this with the exotic incarnation of (the double
1
cover of) J as a quaternionic reflection group (in the first few parts of Sec-
2
tion5.6),and/ortheoctonionicLeechlattice(Section5.6.12).Butonecannot
go far in the study of the sporadic groups without a thorough understanding
of M .
24
I was introduced to the weird and wonderful world of finite simple groups
byacourseoflecturesonthesporadicsimplegroupsgivenbyJohnConwayin
Cambridgeintheacademicyear1978–9.Duringthatcourse,andthefollowing
three years when he was my Ph.D. supervisor, he taught me most of what
subsequently appeared in the ‘Atlas of finite groups’ [28], and a large part of
what now appears in this book. I am of course extremely indebted to him for
this thorough initiation.
Especial thanks go also to my former colleague at the University of Birm-
ingham,Chris Parker,who,earlyin2003,fuelledbyacoupleofpints ofbeer,
persuadedmetherewasaneedforabookofthiskind,andvolunteeredtowrite
half of it; who persuaded the Head of School to let us teach a two-semester
courseonfinitesimplegroupsin2003–4;whodevelopedtheoriginalideainto
a detailed project plan; and who then quietly left me to get on and write the
book. It is not entirely his fault that the book which you now have in your
hands bears only a superficial resemblance to that original plan: I excised the
chapters which he was going to write, on Lie algebras and algebraic groups,
and shovelled far more into the other chapters than we ever anticipated. At
the same time the planned 150 pages grew to nearly 300. Indeed, the more
I wrote, the more I became aware of how much I had left out. I would need
at least another 300 pages to do justice to the material, but one has to stop
somewhere. I apologise to those readers who find that I stopped just at the
point where they started to get interested.
Several colleagues have read substantial parts of various drafts of this
book, and made many valuable comments. I particularly thank John Bray,
whose keen nose for errors and assiduousness in sorting out some of the finer
pointshasimprovedtheaccuracyandreliabilityofthetextenormously;John
Bradley, whose refusal to accept woolly arguments helped me tighten up the
expositioninmanyplaces;andPeterCameronwhosecommentsonsomeearly
draftchaptershaveledtosignificantimprovements,andwhoseencouragement
has helped to keep me working on this book.
I owe a great deal also to my students for their careful reading of various
versions of parts of the text, and their uncovering of countless errors, some
minor,someserious.Iusedtotellthemthatiftheyhadnotfoundanyerrors,
it was because they had not read it properly. I hope that this is now less
truethanitusedtobe.IthankinparticularJonathanWard,JohannaRa¨m¨o,
Simon Nickerson, Nicholas Krempel, and Richard Barraclough, and apologise
to any whose names I have inadvertently omitted.
It is a truism that errors remain, and the fault (if fault there be), human
nature.Byconvention,theresponsibilityismine,butinfactthatisunrealistic.
VIII Preface
As Gauss himself said, “In science and mathematics we do not appeal to
authority, but rather you are responsible for what you believe.” Nevertheless,
Ishallendeavourtomaintainaweb-siteofcorrectionsthathavebeenbrought
tomyattention,andwillbegratefulfornotificationofanyfurthererrorsthat
you may find.
ThanksgoalsotoKarenBorthwickatSpringer-VerlagLondonforhergen-
tlebutpersistentpressure,andtotheanonymousrefereesfortheirenthusiasm
for this project and their many helpful suggestions. I am grateful to Queen
Mary, University of London, for their initially relatively light demands on me
when I moved there in September 2004, which left me time to indulge in the
pleasures of writing. It is entirely my own fault that I did not finish the book
before those demands increased to the point where only a sabbatical would
suffice to bring this project to a conclusion. I am therefore grateful to Jianbei
An and the University of Auckland, and John Cannon and the University of
Sydney, for providing me with time, space, and financial support during the
last six months which enabled me, among other things, to sign off this book.
London Robert Wilson
June 2009
Contents
1 Introduction............................................... 1
1.1 A brief history of simple groups ........................... 1
1.2 The Classification Theorem............................... 3
1.3 Applications of the Classification Theorem.................. 4
1.4 Remarks on the proof of the Classification Theorem ......... 5
1.5 Prerequisites............................................ 6
1.6 Notation ............................................... 9
1.7 How to read this book ................................... 10
2 The alternating groups .................................... 11
2.1 Introduction ............................................ 11
2.2 Permutations ........................................... 11
2.2.1 The alternating groups ............................ 12
2.2.2 Transitivity ...................................... 13
2.2.3 Primitivity....................................... 13
2.2.4 Group actions .................................... 14
2.2.5 Maximal subgroups ............................... 14
2.2.6 Wreath products.................................. 15
2.3 Simplicity .............................................. 16
2.3.1 Cycle types ...................................... 16
2.3.2 Conjugacy classes in the alternating groups .......... 16
2.3.3 The alternating groups are simple................... 17
2.4 Outer automorphisms.................................... 18
2.4.1 Automorphisms of alternating groups ............... 18
2.4.2 The outer automorphism of S ..................... 19
6
2.5 Subgroups of S ......................................... 19
n
2.5.1 Intransitive subgroups............................. 20
2.5.2 Transitive imprimitive subgroups ................... 20
2.5.3 Primitive wreath products ......................... 21
2.5.4 Affine subgroups.................................. 21
2.5.5 Subgroups of diagonal type ........................ 22
X Contents
2.5.6 Almost simple groups ............................. 22
2.6 The O’Nan–Scott Theorem ............................... 23
2.6.1 General results ................................... 24
2.6.2 The proof of the O’Nan–Scott Theorem.............. 26
2.7 Covering groups......................................... 27
2.7.1 The Schur multiplier .............................. 27
2.7.2 The double covers of A and S .................... 28
n n
2.7.3 The triple cover of A ............................. 29
6
2.7.4 The triple cover of A ............................. 30
7
2.8 Coxeter groups.......................................... 31
2.8.1 A presentation of S .............................. 31
n
2.8.2 Real reflection groups ............................. 32
2.8.3 Roots, root systems, and root lattices ............... 33
2.8.4 Weyl groups ..................................... 34
Further reading.............................................. 35
Exercises ................................................... 35
3 The classical groups ....................................... 41
3.1 Introduction ............................................ 41
3.2 Finite fields............................................. 42
3.3 General linear groups .................................... 43
3.3.1 The orders of the linear groups ..................... 44
3.3.2 Simplicity of PSL (q) ............................. 45
n
3.3.3 Subgroups of the linear groups ..................... 46
3.3.4 Outer automorphisms ............................. 48
3.3.5 The projective line and some exceptional isomorphisms 50
3.3.6 Covering groups .................................. 53
3.4 Bilinear, sesquilinear and quadratic forms .................. 53
3.4.1 Definitions....................................... 54
3.4.2 Vectors and subspaces............................. 55
3.4.3 Isometries and similarities ......................... 56
3.4.4 Classification of alternating bilinear forms ........... 56
3.4.5 Classification of sesquilinear forms .................. 57
3.4.6 Classification of symmetric bilinear forms ............ 57
3.4.7 Classification of quadratic forms in characteristic 2.... 58
3.4.8 Witt’s Lemma.................................... 59
3.5 Symplectic groups ....................................... 60
3.5.1 Symplectic transvections........................... 61
3.5.2 Simplicity of PSp (q) ............................ 61
2m
3.5.3 Subgroups of symplectic groups..................... 62
3.5.4 Subspaces of a symplectic space .................... 63
3.5.5 Covers and automorphisms......................... 64
3.5.6 The generalised quadrangle ........................ 64
3.6 Unitary groups.......................................... 65
3.6.1 Simplicity of unitary groups........................ 66